When I lived over a brothel in Verona, I met the assistant restorer of one of the great North American museums, who was staying in a camper-van outside the front door. I mentioned Giovanni Bellini's `Madonna and Child' in his museum: `My boss painted that,' he said. I have long since understood what he meant. Many famous paintings all over the world are essentially the creation of restorers: there can be so little left of an Old Master. That curious picture by Pisanello in the National Gallery, `The Madonna and Child with Sts George and Anthony', is a case in point. It came into the collection in 1867, as a gift from the widow of the late director Sir Charles Eastlake, who had bought it in 1862. Yet Eastlake himself had turned it down a few years earlier because the surface of the picture no longer existed. After the attentions of Giovanni Molteni of Milan, a gifted 'restorer' -'resurrectionist' might be more apt - hey presto! a complete image had appeared. It looks very odd. You would be pushed to find any parallel in 15th-century pictures for the swirls of golden light in the sky that surround the Madonna; understandably, they recall Symbolist works of the end of the 19th century.

It does not do to say so, but there is also something uncomfortably Victorian about several works by Giovanni Bellini, a front-- parlour cosiness suggestive of macassar oil and conservatory plants. The Madonnas are too submissive; and the Christ Child can make your stomach churn. One half expects to see the aspidistra flying in those famous background views of the Veneto: more Baedeker than Bible.A remarkable exhibition in the Venice Accademia shows that we are right to think twice. It lines up a dozen or so of the most famous Madonnas by Giovanni Bellini, together with other works from the Accademia collection. They are shown beneath clinically harsh lighting, like a dentist's surgery, and the fillings are quite easy to spot. The exhibition either tells the truth about paint losses and overpainting, in an astonishing display of candour, with before and after photographs; or it lets the pictures tell their own sad tales.

The fragmentary 'Redeemer', for example, has a surface that is a palimpsest of repaints, which helps to explain its oddly saccharine appearance. The `Madonna with red cherubs' is horrifyingly damaged. The `Madonna del pollice' is an overpainted wreck, which has not been cleaned, presumably for fear that it might completely vanish. It is also odd in being the only panel picture attributed to Bellini painted on pine rather than poplar, and such exceptions should always, as Poirot might say, give you to think. The surface of the Madonna enthroned with sleeping Child is so worn that the image is a mere shadow.

In contrast, the superb under-drawing and under-modelling in the flesh areas of the great `Madonna of the poplars' have become apparent chiefly because of the increased translucency of its top layer of paint, a normal process. It remains a supreme achievement against which less convincing pictures ought to be measured, and attributions such as `The Annunciation' do not pass the test. It is also wonderful to be able to examine in one place four large triptychs that appear to have been produced by an early Bellini workshop. Identifying different hands in them, as with all the works on view, is like the best kind of New Year quiz.

Bellini emerges as consistently tough and uncompromising, compared with both 19th-century restorers and the assistants whose work he sold under his own signature. The presence in this exhibition of the late `Derision of Noah' from Besancon is crucial. Its awkward angularities are mingled with the most sensuous draperies and colouring, and this unsettling combination makes it a moving companion to the haunting `Pieta Dona dalle rose'. Both these autograph masterpieces fit easily with the strange 'Allegories', which can be closely studied for once (the 'Envy' badly worn, the 'Melancholy' in excellent condition). Comparable works in London are the `Death of St Peter Martyr' (Courtauld Gallery) and the earlier `Blood of the Redeemer' (National Gallery). The incisive hand of Bellini can probably be glimpsed in the earlier workshop triptychs, for example, in the Baptist of the St Sebastian Triptych.

This searching exhibition is a rehearsal for an ingenious Bellini 'itinerary', and both are encompassed in an excellent catalogue. Armed with a plan, you can follow a trail around Venice of other works in museums and altarpieces still in their original churches. It takes in such marvels as the `St John Chrysostom' in S Giovanni Crisostomo (don't miss the magical chapel by Tullio Lombardo opposite).

As so often with Venice, riches are piled upon riches and, on the opposite bank of the Canal from the Accademia, Fiat have mounted another of their massive exhibitions devoted to a culture of the distant past, this time the Etruscans: 700 objects in 36 rooms of Palazzo Grassi, with a large and exemplary catalogue to match.

The exhibition makes strenuous efforts to bring the lost world of the Etruscans to life, with the sounds of ignorant armies clashing by night, computer simulations, and mock tombs. The oddest experience is a recording of actors speaking Etruscan: it sounds like nothing on earth, which is only to be expected, since the language is not related to any other. Even through this veil of over-design, the artefacts manage to speak eloquently for themselves.

The Etruscans were masters of gold and bronze but above all of terracotta, and there are lots of wonderful such sculptures on view that once adorned houses and temples, as well as some magnificent life-size marbles of recumbent figures from graves. There are huge bits of fresco, from such locations as the famous Francois Tomb at Vulci, including one sequence that resembles Ronald Searle's cartoon in Molesworth, of soldiers interconnected by swords inserted into each other's bowels. A breathtaking group of small bronzes of the Etruscan gods, some of whom were old, some new, some borrowed from the Greeks, includes a Hercules who, in Etruscan hands, looks oddly like Catwoman.

What happened to the Etruscans? In the 5th century BC, by which time they had been going strong for 500 years, and had ruled Rome under the Tarquins from 690 to 509 BC, their territories to the north in the Po valley and south in Campania contracted. There were battles with Rome in the 4th century, and after 270 BC it was more a matter of conquest by consent and absorption into Rome. The Latin proverb that those we harm, we hate, suggests as much, because the Romans never hated the Etruscans but rather sought to find in them a legitimisation of their own ancestry. They especially valued Etruscan skills in interpreting entrails, and the Etruscans offered the Romans models for both hydraulic engineering and road-building. Towards the end of the Roman Republic, posh Etruscans were keen on getting gongs from Rome, and sending their children to the capital for the best education. Typical colonials, in fact.

II Colore Ritrovato: Bellini a Venezia, at the Accademia, Venice, until 28 January 2001; The Etruscans, Palazzo Grassi, Venice, until 1 July 2001.